U.S. Pat. No. 3,381,058 discloses the use of hydroxyaromatic dicarboxylic acids (e.g. 5-hydroxyisophthalic acid) in preparing polyesters which have improved dyeability with basic and direct dyes. The polyester compositions disclosed in this patent are useful as fibers, but not as resins for producing coatings when crosslinked.
It is known (EP 0,419,088) that polyester resins containing terminal hydroxy groups may be capped by reacting with carboxyphenols (e.g. p-hydroxy benzoic acid) to give terminal groups such as I ##STR1## wherein R.sub.1 is a direct bond, C.sub.1 -C.sub.20 hydrocarbylene or C.sub.1 -C.sub.20 oxyhydrocarbylene and R.sub.2 is hydrogen, hydroxy, halo, C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy or C.sub.1 -C.sub.4 alkoxycarbonyl and that said resins may be crosslinked to produce coatings having enhanced properties. It is noted that the hydroxy functionalized benzoate moieties in these known compositions are monovalent--the hydroxy group is not reactive under the conditions employed for resin synthesis--and always in terminal positions on the polymer chain, in contrast to the hydroxy functionalized aromatic diester moieties of this invention which are divalent which are located randomly along the polymer chain as inherent structural units.
Lastly, it is known (Eastman Kodak Publication No. N-278, "Resin Formulations from Eastman Industrial Chemicals", May 1981) that valuable coatings can be prepared from polyester resins similar to those of this invention but without containing the hydroxy functionalized isophthalate ester moieties, the structural unit which characterizes the valuable resins/coatings of this invention. The previously known coatings have shown deficiencies in the properties of hardness, acid resistance and stain resistance, etc.
There is a need in the coatings industry for high strength, high modulus, chemically resistant coatings. This invention is directed toward meeting the above needs and the coatings described have improvements in one or more specific properties of acid resistance, pencil hardness, impact resistance, solvent resistance, gloss, etc.
The present invention provides polyester resin compositions containing hydroxy functionalized isophthalate moieties, said resins being prepared by a direct polycondensation process which is suitable for economical, large-scale production. These new resins are useful as coatings binders and may be dissolved in conventional solvents and formulated into thermosetting coating compositions which exhibit a good combination of the desirable properties mentioned above.